Specialized small valve systems have been developed to control fluid flow to evacuated chambers such as those used with mass spectrometers, vacuum evaporation and coating systems, epitaxial deposition systems, plasma etching installations, ion milling, sputtering systems, ion implantation, gas mixing and the like. Such valve systems have had to meet the special demands of being capable of controlling fluid flow between areas having immense pressure differentials. Additionally, they have usually had to meet the requirements of having very precise actuator mechanisms, or of being very small, or of being both very precise and very small. Efforts to teach gas flow control systems, actuators and valves of this kind, which are especially suited for use with mass spectrometers, are set forth in U.S. Pat. No. 3,895,231 and its divisional U.S. Pat. Nos. 3,926,209 and 4,018,241. These references disclose systems in which fluid flow to an evacuated chamber, and especially in mass spectrometers, is controlled by utilizing a valve in which a tapered needle is positionally adjusted with respect to an inlet opening or valve seat by a piezoelectric ceramic actuator on which the needle is mounted. The piezoelectric ceramic actuator is flexed by the application of electric potential (voltage). In the systems taught by the above U.S. patents, servo-control systems provide for the selection of the amount of electric potential, which in turn controls the amount of piezoelectric ceramic flexing, and the concommitant movement of the valve needle from the valve seat, and therefore the volume of fluid which can flow through the valve. In those systems, the servo-control systems taught are responsive to some condition, such as pressure, within the chamber into which the fluid is being injected.
In practice, such valve systems have usually required quite small openings, say 0.05 microns or less in diameter, and the valve stem movements have been in the micrometer and nanometer range. In such applications, due to the small sizes and tolerances of such valve systems, and also due to the nature and character of the piezoelectric actuator, such valve systems have been difficult to construct accurately. Once constructed such valve systems have been difficult to maintain. More specifically, in order to reliably provide precise small valve movements of, for example, one micrometer or less by conventional means requires tolerances in the machining and in the assembly of the valve parts which are extremely difficult to obtain. Additionally, at such small dimensions, even with almost perfect machining and assembly, after construction of such a valve, drifts can occur in the piezoelectric actuator, stem, or other components of the system such that the position of the valve stem at the valve opening shifts or the closing force of the valve can be seriously modified. Such drifts or shifts can be as large or even much larger than the intended valve movement.
Such drift may be caused by, for example, temperature variations that occur either from time to time or over a relatively extended interval of time. Drift may also be caused by metal creep, or may be due to any of a variety of other physical phenomena.
At the present time, the foregoing and other problems, when not dealt with, result in loss of control and accuracy of such valve systems. When dealt with, such problems require frequent mechanical and biasing adjustments of the valve system, at a great cost in time as well as in money.